1. Field of the Invention
The present invention relates to a correction based on media white points in case of creating data used in a color matching process.
2. Related Background Art
FIG. 1 is a conceptional view showing a general color matching process executed among different devices. Input data being R (red), G (green) and B (blue) data is transformed to X, Y and Z data on a color space not depending on the device, by an input profile. Since colors other than colors in a color reproduction range (gamut) of an output device can not be represented by the output device, color gamut mapping is executed to the input data to obtain data on the color space not depending on the device such that all of the colors can be included in the color reproduction range of the output device. After executing the color gamut mapping, the input data is transformed from the data on the color space not depending on the device to C (cyan), M (magenta), Y (yellow) and K (black) data on a color space depending on the output device.
In the color matching process, a reference white point and environment light are fixed. For example, in profiles regulated by ICC (International Color Consortium), PCS (Profile Connection Space) for connecting profiles has XYZ values (i.e., X, Y and Z values) and Lab values under the reference of D50. Therefore, correct color reproduction for input originals and print outputs can be guaranteed in case of performing observation under a light source of which characteristic is defined by the reference of D50, and the correct color reproduction can not be guaranteed under a light source of other characteristics.
In case of observing the same sample (e.g., an image) under different light sources, the XYZ values for the sample to be observed are naturally differed in their values. In order to make a prediction of the XYZ values under the different light sources, there are transformation systems such as a prediction expression and the like according to (1) ratio transformation, (2) Von Kries transformation, (3) color perception model, and the like.
The ratio transformation is such a method of performing a ratio transformation of W2/W1 in order to transform XYZ values under a reference white point W1 to XYZ values under a reference white point W2. When this method is adopted to a Lab uniform color space, Lab values under the W1 coincide with Lab values under the W2. For example, when XYZ values for a sample under the W1 (Xw1, Yw1, Zw1) are assumed as XYZ values (X1, Y1, Z1), and XYZ values for a sample under the W2 (Xw2, Yw2, Zw2) are assumed as XYZ values (X2, Y2, Z2), following relationship is obtained according to the ratio transformation.
X2=(Xw2/Xw1)xc2x7X1
Y2=(Yw2/Yw1)xc2x7Y1
Z2=(Zw2/Zw1)xc2x7Z1xe2x80x83xe2x80x83(1) 
The Von Kries transformation is such a method of performing a ratio transformation of W2xe2x80x2/W1xe2x80x2 on a human""s color perception space PQR in order to transform the XYZ values under the W1 to the XYZ values under the W2. When this method is adopted to the Lab uniform color space, the Lab values under the W1 are not coincided with the Lab values under the W2. For example, when the XYZ values for the sample under the W1(Xw1, Yw1, Zw1) are assumed as the XYZ values (X1, Y1, Z1), and the XYZ values for the sample under the W2(Xw2, Yw2, Zw2) are assumed as the XYZ values (X2, Y2, Z2), following relationship is obtained according to the Von Kries transformation.                               [                                                    X2                                                                    Y2                                                                    Z2                                              ]                =                                                            [                                  M                                      -                    1                                                  ]                            ⁡                              [                                                                                                    P2                        /                        P1                                                                                    0                                                              0                                                                                                  0                                                                                      Q2                        /                        Q1                                                                                    0                                                                                                  0                                                              0                                                                                      R2                        /                        R1                                                                                            ]                                      ⁡                          [              M              ]                                ⁡                      [                                                            X1                                                                              Y1                                                                              Z1                                                      ]                                              (        2        )            
where,       [                            P1                                      Q1                                      R1                      ]    =                              [          M          ]                ⁡                  [                                                    Xw1                                                                    Yw1                                                                    Zw1                                              ]                    ⁢              
            [                                    P2                                                Q2                                                R2                              ]        =                                        [            M            ]                    ⁡                      [                                                            Xw2                                                                              Yw2                                                                              Zw2                                                      ]                          ⁢                  
                [        M        ]            =                                    [                                                            0.40024                                                  0.70760                                                                      -                    0.08081                                                                                                                    -                    0.22630                                                                    1.16532                                                  0.04570                                                                              0                                                  0                                                  0.91822                                                      ]                    ⁢                      
                    [                      M                          -              1                                ]                =                  [                                                    1.85995                                                              -                  1.12939                                                            0.21990                                                                    0.36119                                            0.63881                                            0                                                                    0                                            0                                            1.08906                                              ]                    
The prediction expression according to the color perception model is a method of performing transformation by utilizing, for example, a human""s color perception space QMH (or JCH) such as a space related in the CIECAM97s in order to transform XYZ values under an observation condition VC1 (including W1) to XYZ values under an observation condition VC2 (including W2). Where, symbol Q in the abbreviation QMH denotes brightness, symbol M denotes colorfulness and symbol H denotes huequadrature or hueangle. Symbol J in the abbreviation JCH denotes lightness, symbol C denotes chroma and symbol H denotes huequadrature or hueangle.
For example, when the XYZ values for the sample under the W1(Xw1, Yw1, Zw1) are assumed as the XYZ values (X1, Y1, Z1), and the XYZ values for the sample under the W2(Xw2, Yw2, Zw2) are assumed as the XYZ values (X2, Y2, Z2), the following transformation is performed according to the color perception model.
That is, (X1, Y1, Z1)xe2x86x92[CIECAM97s forward transformation]xe2x86x92(Q, M, H) or (J, C, H)xe2x86x92[CIECAM97s inverse transformation]xe2x86x92(X2, Y2, Z2)xe2x80x83xe2x80x83(3) 
However, the above description depends on a case that the sample is represented on ideal media (white point on medium corresponds to complete reflection and black point on medium corresponds to complete absorption), while the media actually used have different conditions.
For example, since colors displayed on a monitor are represented by light source colors, a white point on medium for color data of R=G=B=225 can be considered as relative brightness of Y=100%, however, a black point on medium for color data of R=G=B=0 does not correspond to relative brightness of Y=0%. Since color of printed matter corresponds to color of the object (or objective color), whiteness in paper represented by C=M=Y=K=0% has a certain reflection ratio xcex3, therefore, the relative brightness of Y=100% can not be obtained. Even color represented by C=M=Y=K=100% being the most dark color is not resulted in the relative brightness of Y=0%.
Since the whiteness in paper of the printed matter does not have characteristic of complete reflection, it does not coincide with the white point (e.g., D50) of light source to be irradiated. Similarly, black (C=M=Y=K=100%) available in a device does not exist on a gray axis of the light source (chroma identical to that of the light source) to be irradiated.
Hereinafter, a white point (color corresponding to R=G=B=255 in an RGB device, and corresponding to C=M=Y=K=0% in a CMYK device) represented on medium of each device is defined as xe2x80x9cmedia white pointxe2x80x9d. A black point (color corresponding to R=G=B=0 in the RGB device, and corresponding to C=M=Y=K=100% in the CMYK device) represented on medium of each device is defined as xe2x80x9cmedia black pointxe2x80x9d.
As in the above description, since xe2x80x9cwhite point of light sourcexe2x80x9d, xe2x80x9cmedia white pointxe2x80x9d, xe2x80x9cblack point of light sourcexe2x80x9d and xe2x80x9cmedia black pointxe2x80x9d are different, in case of performing comparison of images on different media under the same light source, there sometimes occurs a case that impressions of those images are different from each other. For example, in a case where white points on media are different from each other such as white points on a blank (white) paper and newspaper (or a recycled paper), when those media are laterally arranged for the comparison, it is obviously recognized by a person that the whiteness in paper are different from each other. However, in case of individually performing the comparison for each of media, even if a media white point is considerably different from other media white points, like the newspaper, the person perceives the whiteness on a paper as xe2x80x9cwhitexe2x80x9d. This phenomenon occurs due to a fact that human""s visual perception is adapted to the color of white. And, if the XYZ values under the same light source are only to be coincided, the XYZ values for the color to be printed may be coincided, however, the whiteness in paper in a background is remained in different state, and there is possibility of overlapping the color on a white background.
Similarly, with respect to the media black point, the black points represented on media are different from each other according to each device. In case of individually performing the comparison for each of media, even if the black points on media are different from each other, the person perceives the blackness on the paper as xe2x80x9cblackxe2x80x9d. Therefore, it is considered that the human""s visual perception is also adapted to the color of black. Problems in the color matching process executed among devices having different color reproduction ranges can be avoided by the color gamut mapping technology. However, with respect to a dark portion adjacent to the black point, there sometimes occurred blocked up color (or broken color) even if the color gamut mapping technology is used.
In order to solve these problems, it is required to execute the color matching process not only coinciding the XYZ values under the same light source but also considering the media white point and the media black point.
In the conventional white point correction and black point correction performed on media, there utilized a simple method of executing a scaling process in accordance with brightness (or reflection ratio) after correcting the XYZ values of the sample by using the black point. That is, when a white point on medium is assumed as MW1(Xmw1, Ymw1, Zmw1) and a black point on medium is assumed as MK1(Xmk1, Ymk1, Zmk1), the relationship between the sample (X1, Y1, Z1) on medium and the correction sample (X2, Y2, Z2) was indicated as follows:
xe2x80x9cWhite Point Correction and Black Point Correction on Mediaxe2x80x9d
X2=(X1xe2x88x92Xmk1)/(Ymw1xe2x88x92Ymk1) 
Y2=(Y1xe2x88x92Ymk1)/(Ymw1xe2x88x92Ymk1) 
Z2=(Z1xe2x88x92Zmk1)/(Ymw1xe2x88x92Ymk1)xe2x80x83xe2x80x83(4) 
In the profile regulated by the ICC, in case of representing the XYZ values for the sample on medium on the PCS (D50), it is recommended that the XYZ values are to be relative relation to the media white point. However, that transformation method is not yet established, and a simple method as mentioned below has been used (ICC specification). That is, when a white point on medium is assumed as MW2(Xmw2, Ymw2, Zmw2), and a white point of the light source on the PCS is assumed as IW3(Xiw3, Yiw3, Ziw3), the relationship between the sample (X2, Y2, Z2) on medium and a sample (X3, Y3, Z3) on the PCS was indicated as follows:
xe2x80x9cCorrection from White Points on Media to White Points on PCS (D50)xe2x80x9d
X3=(Xiw3/Xmw2)xc2x7X2
Y3=(Yiw3/Ymw2)xc2x7Y2
Z3=(Ziw3/Zmw2)xc2x7Z2xe2x80x83xe2x80x83(5) 
However, since the expression (4) and the expression (5) are both based on the ratio transformation on a XYZ color space, the obtained result was not fitted to the human""s color perception. Furthermore, the expression (4) has a defect that chromaticity of the white point is varied due to the XYZ values of the black point.
That is, in the color matching process executed among media having different white points and different black points, there were the following problems:
how the color matching process executed among the media having different white points is to be treated; and
how the color matching process executed among the media having different black points is to be treated.
An object of the present invention is to solve these problems.
To achieve the above object, the present invention has the following constitution.
An image processing method of creating transformation data used in transformation performed between data on a color space depending on a device and data on a color space not depending on a device is characterized in that a colorimetry value of a sample is corrected on a human""s color perception space on the basis of a media white point, and the transformation data is created on the basis of the corrected colorimetry value.
An image processing method of creating transformation data used in a color matching process executed between a source device and a destination device is characterized in that a white point correction for transforming colors on media at the source device side to colors on media at the destination device side is performed on a human""s color perception space on the basis of white points on media at the source device side and white points on media at the destination device side.
A recording medium for recording a program of realizing an image processing method of creating transformation data used in transformation performed between data on a color space depending on a device and data on a color space not depending on a device is characterized in that a colorimetry value of a sample is corrected on a human""s color perception space on the basis of a media white point, and a program for creating the transformation data on the basis of the corrected colorimetry value is recorded.
A recording medium for recording a program of realizing an image processing method of creating transformation data used in a color matching process executed between a source device and a destination device is characterized in that a program for performing a white point correction for transforming colors on media at the source device side to colors on media at the destination device side on a human""s color perception space on the basis of white points on media at the source device side and white points on media at the destination device side is recorded.
The above and other objects of the present invention will become apparent from the following detailed description in conjunction with the attached drawings.